1. Field of the Invention
The present invention relates to an apparatus and method for fabricating fiber-optic gratings, and in particular, fabricating fiber-optic gratings over a wide variety of Bragg wavelengths and bandwidths.
2. Background of the Invention
Fiber-optic Bragg gratings have become important components in modern telecommunication systems and fiber sensing systems. A fiber-optic Bragg grating consists of a longitudinal, periodic variation in the refractive index in the core of an optical fiber. When light propagates through a fiber grating, Bragg diffraction causes one wavelength to be selectively reflected. The wavelength at which high reflectivity occurs is determined by the period of the grating.
A grating writing system imparts a desired Bragg grating onto an optical fiber. One of the considerations for the grating writing system is its capability of fabricating gratings over a large wavelength range in a precisely controlled manner.
Fiber-optic Bragg grating is typically produced by exposing the optical fiber to an intense ultraviolet (UV) interference pattern of two interference UV beams. The resulting Bragg grating will have a period which is the same as that of the UV interference pattern. It is critical to precisely control the angle between the two interference UV beams for precision fabrication of the fiber gratings.
Two conventional techniques which utilize an UV interference pattern to produce fiber gratings with the necessary accuracy include an interferometric method and a phase-mask method. In the interferometric method, a single laser beam is split into two components, which are subsequently recombined at the optical fiber to produce an interference pattern. The period of the pattern is controlled by rotating two mirrors.
The phase-mask method utilizes a diffraction grating to split a single laser mainly into +1/−1 diffractive orders. Interference between the two first diffractive orders creates a fixed pattern in the optical fiber. The period of the pattern is determined discretely by using different phase masks.
A disadvantage of the interferometric method is that this method tends to be complicated to implement and requires a high degree of optical alignment, mechanical stability, and precision adjustment. A disadvantage of the phase-mask method is that this method does not have the flexibility of tuning a Bragg wavelength. In order to vary the Bragg wavelength written to an optical fiber, a different phase mask is required for each Bragg wavelength.
An additional method for fabricating an optical fiber grating is provided by U.S. Pat. No. 5,912,999 to Brennan, III et al. (hereinafter “Brennan”) which uses a writing beam to form a periodic intensity distribution of period Λ while the optical fiber is translated relative to the intensity distribution at a velocity v(t). Unlike an interference pattern written method, the period of the fiber Bragg grating is not determined by the interference pattern from the phase mask.
In the Brennan method, the period of the fiber Bragg grating is determined by the frequency of the laser intensity modulation and the velocity by which the fiber is translated. Since the precision of the Bragg wavelength various inversely with the size of the beam, the smaller the beam size, the more precise the fiber Bragg grating wavelength imparted. Therefore, the beam size has to be very narrow in order to produce a desired and an accurate grating. For example, Brennan teaches a beam diameter of less than approximately ten microns.
A disadvantage with the Brennan method is that the movement of the optical fiber perpendicular to the writing beam may cause vibration to the optical fiber thereby affecting the fiber Bragg grating applied thereto. An additional disadvantage with the Brennan method is that the method and apparatus require a high degree of precision in optical fiber movement control.
Yet another method for fabricating a fiber Bragg grating is that of U.S. Pat. No. 6,072,926 to Cole et al. (hereinafter “Cole”) which is directed to forming optical waveguide gratings using a writing laser to generate a grating pattern on an optical fiber while moving the optical fiber perpendicular to the writing beam. The period of the fiber Bragg grating is varied by smearing the interference pattern. A phase mask is used to form an interference pattern.
One disadvantage with the Cole method is that the grating strength tends to decrease as the index of modulation gets averaged or “washed out” when the optical fiber moves too quickly through the interference pattern formed by the phase mask. Therefore, the Cole method has a limited range of Bragg wavelength shift. In addition, like the Brennan method, since the optical fiber is moving perpendicular to the writing beam while the fiber Bragg grating is being written, vibration of the optical fiber may affect the accuracy of the fiber Bragg grating produced.
An additional method for fabricating a fiber Bragg grating is provided by WO 99/22256 to Lamin et al. (hereinafter “Lamin”) which uses a converging optical system comprising a positive lens downstream of a phase mask and which adjusts the period of the fiber Bragg grating by moving either or both the optical fiber or the lens. Lamin teaches that tuning of the period of the interfering beams and hence the resulting Bragg wavelength, can be achieved by varying the distance between the lens and the phase mask. A disadvantage with Lamin is that the Lamin method is very sensitive to fiber position due to the nature of its converging optical system.